1. Field of the Invention
The present invention relates to a sheet supplying apparatus for supplying a sheet such as a recording sheet or an original sheet to an image forming apparatus such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
In the past, for example, in an image forming apparatus such as a facsimile system and the like, sheets such as original sheets supplied from a sheet stacking tray are separated one by one by a separation means, and a separated sheet is sent, by a convey means, to a reading portion where image information on the sheet is read, and thereafter the sheet is discharged out of the apparatus.
The sheets stacked on the sheet stacking tray are sent to the separation means from the lowermost one by rotating an auxiliary convey means (referred to as "auxiliary convey roller" hereinafter) disposed below the sheet stacking tray while abutting it against the lowermost sheet of the sheet stack. Since a rotary shaft of the auxiliary convey roller is not shifted and a peripheral surface of the auxiliary convey roller has an ellipic shape or a triangular shape in order to transmit and interrupt the sheet feeding force to the sheet, when the auxiliary convey roller is rotated, the peripheral surface of the auxiliary convey roller is protruded upwardly and retracted downwardly with respect to the sheet stacking tray repeatedly.
On the other hand, there is a conveying mechanism in which a shaft of an auxiliary convey roller can be shifted in an up-and-down direction so that, when it is shifted upwardly, the auxiliary convey roller is abutted against the sheet stack, thereby feeding the sheet. In this mechanism, the upward and downward movement of the auxiliary convey roller is controlled by an electric signal via a plunger or an electromagnetic clutch for shifting the auxiliary convey roller.
However, in case of the auxiliary convey roller having the elliptic or triangular peripheral surface, since the auxiliary convey roller is extended and retracted with respect to the sheet stacking tray, the feeding force is transmitted and interrupted repeatedly with respect to the sheet (i.e. transmitted intermittently). Thus, it takes a long feeding time and the influence upon the-separation means is not stabilized, thereby causing the double-feed of the sheets or the poor sheet feeding.
On the other hand, in the mechanism for controlling the upward and downward movement of the auxiliary convey roller by using the plunger or the electromagnetic clutch, since the number of parts is increased, the apparatus is made expensive and large-sized.